1. Field of the Invention
The present invention relates to the lubricating oils used to lubricate engines run on biodiesel fuels and to the improvement in resistance to oxidation of such lubricating oils.
2. Description of the Related Art
Several types of biodiesel fuels have been proposed for as well as introduced into the diesel fuel blend pool for use in commercial and passenger vehicles. The biodiesel fuels would be used as the exclusive fuel or as an addition to hydrocarbon-based diesel fuels. When used as an addition to hydrocarbon-based diesel fuels, the biodiesel fuels constitute anywhere from 2 to 50 wt % of the resulting diesel fuel blends, preferably 5 to 30 wt % of the blend. In Europe biodiesel fuels either are being considered or already have been mandated for use in hydrocarbon-based diesel fuels in an amount in the range of 5 to 10 wt %.
Biodiesel fuels are being considered as alternatives to hydrocarbon-based diesel fuels or as diesel fuel blend pool components because of their derivation from renewable plant and animal oils.
Biodiesel fuels are mixtures of lower, short chain esters of mixed saturated and unsaturated straight chain fatty acids derived from vegetable and/or animal fats and oils. The straight chain fatty acids are, typically, C10 to C26 fatty acids, preferably C12 to C22 fatty acids. The fatty acids are made into biodiesel by trans-esterification using short chain alcohols; e.g., C1 to C5 alcohols, in the presence of a catalyst such as a strong base.
Vegetable and/or animal oils and fats are natural triglycerides and are renewable sources of starting material. Typical vegetable oils are soybean oil, rapeseed oil, corn oil, jojoba oil, safflower oil, sunflower seed oil, hemp oil, to coconut oil, cottonseed oil, sunflower oil, palm oil, canola oil, peanut oil, mustard seed oil, olive oil, spent cooking oil, etc., without limitation. Animal fats and oils include beef, pork, chicken fat, fish oil and oil recovered by the rendering of animal tissue.
Plant source biodiesel fuels are currently the more dominant type in the marketplace. The primary plant sources are soy in North America, rapeseed in Europe, and palm and the other plant source oils elsewhere.
The biodiesel is made by esterifying one or a mixture of such oils and fats using one or a mixture of short chain; e.g., C1 to C5, alcohols, preferably methanol.
Because the most economical trans-esterification processes are performed using methanol, the biodiesel products are identified with reference to the oil or fat source; e.g., soy methyl ester (SME), rapeseed methyl ester (RME), etc.
Trans-esterification is effected by the base catalyzed reaction of the fat and/or oil with the alcohol, direct acid catalyzed esterification of the oil and/or fat with the alcohol, or conversion of the oil and/or fat to fatty acids and then to alkyl esters with alcohol in the presence of an acid catalyst. In base catalyzed trans-esterification, the oil and/or fat is reacted with a short chain alcohol, preferably methanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide to produce glycerin and short chain alkyl esters. The glycerin is separated from the product mixture and biodiesel is recovered. Any unreacted alcohol is removed by distillation. The recovered biodiesel is washed to remove residual catalyst or soap and dried.
Because of the natural sources of the oils and/or fats upon which the biodiesel fuels are based, the biodiesel molecules are mixtures of various to molecular weights with ester functionality and up to two olefinic double bonds.
The presence of the olefinic double bonds and ester functionality in the biodiesel fuels results in the biodiesel fuels being susceptible to oxidative degradation, resulting in the unsuitability of biodiesel for long term storage.
Further, the instability of ester and olefinic double bonds in the biodiesel fuels also is a source of oxidative instability of the lubricating oil used to lubricate biodiesel fueled engines, the lubricating oil being rendered more susceptible to sludge and deposit formation.
The improvement in the oxidation stability of biodiesel fuel has been the subject of investigation leading to the addition to such fuel of various additives and combinations of additives to effect the desired stabilization.
WO 2008/056203 teaches stabilizer compositions for blends of petroleum and renewable fuels. Mixtures of renewable fuels such as biodiesel, ethanol and biomass mixed with conventional petroleum fuel are stabilized by the addition thereto of a multifunctional additive package which is a combination of one or more additives selected from the group consisting of a free radical chain terminating agent, a peroxide decomposition agent, an acid scavenger, a photochemical stabilizer, a gum dispersant and a metal sequestering agent. Peroxide decomposition agents are selected from the group containing sulfur, nitrogen and phosphorus compounds. Suitable nitrogen-containing compounds are of the general formula:
wherein R, R′ and R″ can be alkyl linear, branched, saturated or unsaturated C1-C30, aromatic, cyclic, poly alkoxy, polycyclic. Identified as a useful nitrogen-containing compound is N—N_dimethylcyclohexylamine. While N,N-dimethylcyclohexylamine is taught as a useful peroxide decomposition agent, in the examples it is never employed by itself but always in combination with a phenolic anti-oxidant. Reference to FIG. 2 of WO 2008/056203 reveals that whereas the use of the combination of 75% phenol and 25% N,N-dimethylcyclohexylamine (at a treat level of 200 mg/l) resulted in an improvement in the relative stability of the fuel as compared to using 100% phenol over all time periods tested, an increase in the amount of N,N-dimethylcyclohexylamine in the additive mixture to 50% significantly reduced the beneficial effect of the additive mixture (still at a treat level of 200 mg/l) in terms of relative stability over all time periods tested as compared to the 75% phenol/25% N,N-dimethylcyclohexylamine mixture with the most significant reduction in benefit being observed over the long term; i.e., at the six hour time period.
U.S. 2004/0152930 teaches stable blended diesel fuel comprising an olefinic diesel fuel blending stock containing olefins in an amount of 2 to 80 wt %, non-olefins in an amount of 20 to 98 wt % wherein the non-olefins are substantially comprised of paraffins, oxygenates in an amount of at least 0.012 wt % and sulfur in an amount of less than 1 ppm, the blend diesel being stabilized by an effective amount of a sulfur-free anti-oxidant. An effective amount of sulfur-free anti-oxidant is identified as 5 to 500 ppm, preferably 8 to 200 ppm of additive.
The sulfur-free anti-oxidant is selected from the group consisting of phenols, cyclic amines and combinations thereof. Preferably the phenols contain one hydroxyl group and are hindered phenols. The cyclic amine anti-oxidants are amines of the formula:
wherein A is a six-membered cycloalkyl or aryl ring, R1, R2, R3 and R4 are independently H or alkyl and X is 1 or 2. An example of the sulfur-free anti-oxidant is given as di-methylcyclohexylamine. See also U.S. Pat. No. 7,179,311.
“Evaluation of the Stability, Lubricity and Cold Flow Properties of Biodiesel Fuel”, J. Andrew Waynick, 6th International Conference on Stability and Handling of Liquid Fuel”, Vancouver, B.C., Canada, Oct. 13-17, 1997, pages 805-829 addresses various aspects of biodiesel fuel and reports an example where a blend of 80% low sulfur No. 2 diesel fuel/20% methyl soyate ester biodiesel fuel was combined with 20 ppm N,N-dimethylcyclohexylamine. At page 813 the report states that “although additive C (the N,N-dimethylcyclohexylamine) did not control hydroperoxide or insolubles formulations, it did hold the TAN to a level near that of the fuel blend with anti-oxidant additive A (N,N-di-sec-butyl-p-phenylenediamine) and B (2,6-di-t-butyl-4-methyl phenol)”.
U.S. 2008/0127550 discloses stabilized biodiesel fuel composition wherein the stabilizing agent is a combination of: i) one or more compounds selected from the group consisting of sterically-hindered phenolic anti-oxidants; and ii) one or more compounds selected from the group consisting of triazole metal deactivators.
U.S. 2007/0151143 discloses a stabilized biodiesel wherein the stabilizing additive is selected from one or more of the group consisting of the 3-arylbenzofuranones and the hindered amine light stabilizers and, optionally, one or more hindered phenolic anti-oxidants.
U.S. 2007/0248740 discloses an additive composition comprising 2,5-di-tert-butyl hydroquinone (BHQ), N,N′-disalicylidenepropylenediamine. The additive is used to stabilize fuel containing at least 2% by weight of an oil derived from plant or animal material.
U.S. Pat. No. 3,336,124 discloses stabilized distillate fuel oils and additive compositions for such fuel oils. One additive composition comprises a mixture of: (a) an oil soluble dispersant terpolymer of a particular type; (b) from 0.2 to about 3 parts by weight per part of said oil soluble dispersant tripolymer of N,N-dimethylcyclohexylamine; and (c) a normally liquid inert hydrocarbon carrier solvent in an amount to constitute from about 20% to 80% by weight of the additive composition. See also GB 1,036,384.
WO 2008/124390 discloses a synergistic combination of a hindered phenolic anti-oxidant and a detergent to improve the oxidation stability of biodiesel fuel.
While this reference purports to teach a synergistic mixture of a detergent and a hindered phenol anti-oxidant, the detergent is not any of the metal salt type such as alkali or alkane earth metal sulfonates, phenates, carboxylate or salicylate, but, rather, nitrogen-containing detergents such as hydrocarbyl substituted arylated nitrogen compounds (e.g., polyisobutylene succinic anhydride polyamine, i.e., PIBSA-PAM), hydrocarbyl substituted amines (e.g., polyisobutylene amine), and Mannich base-type detergents which are the reaction products of a hydrocarbyl-substituted phenol, an amine and formaldehyde.
U.S. 2007/0289203 is directed to a synergistic combination of anti-oxidants for biodiesel fuels. The synergistic combination is a mixture of a certain aminic anti-oxidant in combination with a phenolic anti-oxidant. While the optional presence of additional components such as detergents is recited at para. [0038], no specific teaching appears to have been made regarding salicylate or phenates nor to any premixing of the components.
WO 2008/121526 is directed to anti-oxidant blends in biodiesel. The anti-oxidant blend is a combination of (1) mono- or bis-hindered phenols derived from 2,6-di-tert butylphenol, and (2) N,N′-disubstituted paraphenylene diamine.
U.S. 2007/0113467 is directed to biodiesel fuel of improved oxidation stability comprising biodiesel fuel and at least one anti-oxidant, the anti-oxidant being selected from the specific group recited at paras. [0006] to [0012]. The possible presence of other additives in the biodiesel is mentioned at para. [0052], such other additives including but not being limited to cetane improvers, ignition accelerator agents, metal deactivators, cold flow improvers, etc. Detergents are recited at para. [0065], but are of the PIBSA-PAM and Mannich base variety. No mention is made of alkali or alkaline earth metal salicylates or phenates nor of the desirability that these detergents be of higher TBN or used as premixes with phenolic and/or aminic anti-oxidants.
U.S. 2008/0182768 is directed to a lubricant composition for biodiesel fuel engine applications. The lubricant contains a major amount of a lubricating oil and a minor amount of a highly grafted multifunctional olefin copolymer, the multifunctionality being derived from the presence of amine moiety on the copolymer (para. [0058] to [0071]). The presence of a DI package is mentioned at para. [0085], the detergent including a metal-containing ash-forming detergent, preferably overbased (TBN 150 or greater) which can be sulfonate, phenate, sulfurized phenate, thiophosphonate, salicylate, naphthenate or other is oil-soluble carboxylates of alkali or alkaline earth metal. See para. [0086].
“Examples” are mentioned at para. [0123] but there appears to be no mention of any detergents at all being used in the Examples.
U.S. 2008/0127550 stabilizes biodiesel fuel by adding to it an effective amount of a combination of one or more stearically hindered phenols and one or more triazole metal deactivators. No mention appears to be made regarding detergents, but materials such as copper naphthenate, copper acetate, iron naphthenate are disclosed in the Examples.
No mention appears to be made regarding alkali or alkaline earth metal salicylates, phenates, carboxylates and/or sulfonates, nor of the TBN of such detergents nor of their use in combination with phenolic and/or aminic anti-oxidants, or as premixes.